1. Field of the Invention
The present invention relates to communications systems, and more particularly, to a method and apparatus for monitoring the quality and integrity of such system at selected points from a central position.
2. Description of the Prior Art
In communications systems where data (digital or analog) are sent from a central point to a number of terminal points for the reliable transmission of information, it is desirable to easily and conveniently monitor the performance of the system on a routine basis. To accomplish this requires the monitoring of system performance at a number of terminal points. In a cable television system, for example, it is desirable to monitor the end of each main cable or trunk as well as the ends of each of the branch or feeder cables. In addition, it is desirable to monitor the condition of the system at the location of intermediate amplifiers, which are known to be prone to faults and malfunctions. Since a cable television system may have several trunk lines, each supplying many feeder lines, the number of monitoring networks which are required for full system observation is extremely large. Therefore, it is quite important to keep the costs of the monitoring circuits to a minimum and to assure reliability and ease of maintenance.
In the development of cable television systems, it was initially recognized that the financial outlay for monitoring equipment would be extremely high and, at the outset, could not be justified. Thus, during the early stages of cable television development, system monitoring was accomplished by the cable subscribers themselves. While this was satisfactory, in the sense that the cable operators were notified of system faults without having to bear the expense of separate monitoring equipment, it was highly unsatisfactory in that there was no way of protecting the consumer from exposure to fault situations. In fact, it not only exposed the consumer to frequent programming disruptions, but obligated the consumer to complain before a fault would be corrected.
As cable systems increased in popularity, and technological innovations brought improvements throughout the system, approaches were taken to provide system monitoring independent of the consumer. Generally speaking, the approaches most often considered in the prior art were based on the periodic transmission of test signals through the system for monitoring by technicians at different locations. This allowed cable operators to reliably monitor cable operation but presented a different set of disadvantages. The primary disadvantage was that manpower was required to travel to each cable terminal point, to tap into the cable, and to take readings for on-the-spot or subsequent analysis. Moreover, since the test signals had to be transmitted through the system for the duration of the testing procedure, interference with normal communications often resulted, so that the entire testing operation had to be restricted to early morning hours before the start of the broadcast day.
Recent developments have shown that coaxial cable systems are capable of providing two-way communications between a central point (headend) and each terminal point. For example, in addition to the RF carriers sent out from the central point to the terminal points, data can be sent back to the headend over the cable system from the terminal points. By convention, communications from the central point to a number of terminal points are called forward direction communications. Likewise, communications from terminal points back to the central point are called reverse communications. Most two-way CATV systems employ a single cable for communications in both the forward direction and the reverse direction by utilizing spectrum splitting filters where the high end of the spectrum carries the forward direction communications and the low end of the spectrum carries the reverse communications. However, there are CATV systems that employ separate coaxial cables routed parallel to each other, where one is used for forward direction and another for reverse directions. In addition, combinations of the aforementioned methods are sometimes employed.
The advent of such two-way CATV systems has opened the door to central monitoring and, thus, has rekindled interest in this area. Attempts have been made to monitor the status of the system at various terminal points by utilizing the two-way capabilities of the system to transmit operational data in the reverse direction for analysis. This has presented several serious obstacles. One of these obstacles is the fact that test signals sent in the forward direction through the system cause interference with normal communications and information signals. Consequently, specialized forms of test signals have been proposed, but the resultant return signals often do not carry enough information to enable full analysis and monitoring of system operation. Another obstacle is the fact that several monitoring circuits are necessary at all of the various terminal points with the result that the number of reverse direction signals becomes difficult to efficiently manage and separately identify without resulting in circuit complexity of prohibitive cost.
U.S. Pat. No. 3,287,715 discloses an indicator system in which a continuous wave oscillator is amplitude modulated by a signal proportional to a locally measured variable. Each remote unit in the system has a predetermined assigned frequency, and all units can be scanned by sequencing from one frequency to the next. The patent does not disclose a system which can detect RF signals as they are actually received at different points for monitoring, but rather, more closely resembles conventional telemetry equipment well known in the art.
An example of such a telemetry-type system is shown in U.S. Pat. No. 3,289,078. In this patent, a measured variable is used to amplitude modulate an oscillator, and the output of the oscillator is transmitted to a central or control location in response to a designated command. This patent does not disclose a system for monitoring the downstream output of a communications system and transmitting such output upstream for analysis.
U.S. Pat. No. 3,651,403 discloses a simultaneous sweep testing system for cable TV. In this system, the test signal applied to the cable is in the form of a series of discrete frequency-swept pulses of short duration. These signals do not interfere with the information signals appearing in the system, but actual examination of signals must still take place at the physical location of each monitor. The patent discloses no circuitry for modulation and retransmission of signals upstream for analysis at a central point.
Another patent which shows the transmission of a swept test signal along a cable network is U.S. Pat. No. 3,978,282. This patent is limited to its disclosure of the test signal transmission network and does not describe circuitry for providing reverse direction transmission of swept signals for headend analysis.
U.S. Pat. No. 4,031,543 discloses a pay TV system with provisions for monitoring cable quality. The system relies on the random transmission of remote signals which are subsequently received and analyzed by computer at the headend.
While the prior art, as exemplified by the above patents, has attempted several solutions to the problem of economical and reliable communications systems monitoring, a fully satisfactory system heretofore has not been developed.